Radiotherapeutic apparatus

ABSTRACT

A radiotherapeutic apparatus comprises a source of therapeutic radiation, a source of visible light arranged to cast a light field corresponding to the beam of radiation, and a multileaf collimator for shaping the beams, wherein a filter is disposed in the path of the visible light beam having a plurality of linear dark sections corresponding to leaves of the collimator. This prevents the incident light from falling on the leaves and removes the ghost images at source. By placing the filter in the head, the line can be very narrow and will be blurred into penumbra at the isocentre. This is therefore a very inexpensive yet effective method of reducing ghosting. A mirror can deflect the path of the visible light to correspond to that of the radiation beam, and the filter can be disposed anywhere in the beam path, such as prior to the mirror, subsequent to the mirror and prior to the collimator, or subsequent to the collimator. The filter thus creates dark sections in the light field corresponding to leaves (preferably all the leaves) of the collimator.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/EP2006/010778, filed Nov. 10, 2006 andpublished as WO 2008/055531 A1 on May 15, 2008, the content of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Radiotherapy works by directing a beam of harmful radiation towards thesite of (for example) a tumour. The radiation inflicts damage on thetumour and causes its reduction. In order to prevent collateral damageto the healthy tissues surrounding the tumour, the beam will be shapedto reduce the dose applied outside the tumour, for example by conformingto the outside shape of the tumour. It will also (generally) be directedtowards the tumour from a variety of different directions along axesthat are centred on the tumour. Thus, by rotating the source around thepatient and varying the shape of the tumour, a three-dimensional dosedistribution can be built up which is at a maximum within the tumour andis minimised elsewhere.

It is therefore important to ensure that the patient is correctlypositioned within the apparatus. The apparatus will work to its own setof co-ordinate axes and will expect the tumour to be positioned in thecorrect location at its “isocentre”, the point in space about which theradiation source rotates and which is therefore always on the radiationaxis. Fine control of the patient position can be achieved by providingan articulated couch for the patient, and modern couches are able toperform adjustments to the patient position in all six degrees offreedom. This is controlled in response to data obtained from adiagnostic x-ray source integrated with the radiotherapeutic apparatus,which can provide real time information as to the current position ofthe patient.

However, it is necessary to ensure that the patient position isapproximately correct before such apparatus can be used to fine-tune theposition of the patient. The initial positioning of the patient to anaccuracy of a few millimeters is therefore assisted by providing a lightsource within the apparatus, together with one or more mirrors (asnecessary) to direct the light beam along the path of the radiation.Cross-hairs within the beam path can be used to align the patient, andthe light field can be used to check operation of the variouscollimators that are provided in order to limit the shape of theradiation beam.

One such collimator is the so-called “multi-leaf collimator” (MLC), asshown (for example) in EP-A-0,314,214. This consists of a plurality ofleaves that can be moved into and out of the radiation path; each leafhas a sufficient depth along the radiation axis to absorb the incidentradiation, and a narrow width transverse to the radiation axis. A largenumber of such leaves are placed alongside each other in two opposingbanks, and each can be moved independently so that they can(collectively) define an arbitrary edge to the radiation field.

One difficulty that can arise when the MLC is being tested with theoptical light source is that the significant depth of the leaves alongthe beam axis and at a very shallow angle thereto allows them to reflectthe incident light. Where a particular leaf is extended or withdrawnsignificantly beyond its adjacent leaves, such reflective surfaces arecreated in the beam path and lead to the phenomenon of “ghosting”,whereby a spurious bright line is created in the field. Previous effortsto eliminate ghosting have relied on surface treatment of the leaves toreduce their reflective properties, but the very shallow angle at whichthey are presented to the light source makes this difficult.

SUMMARY OF THE INVENTION

The present invention therefore provides a radiotherapeutic apparatuscomprising a source of therapeutic radiation, a source of visible lightarranged to cast a light field corresponding to the beam of radiation,and a muitileaf collimator for shaping the beams, wherein a filter isdisposed in the path of the visible light beam, having a plurality oflinear dark sections corresponding to leaves of the collimator.

A mirror can deflect the path of the visible light to correspond to thatof the radiation beam. This allows the visible light source to bedisposed out of the radiation beam, and for the source of visible lightto be disposed in substantially the same optical location as the sourceof therapeutic radiation.

The filter can be disposed anywhere in the beam path, such as prior tothe mirror, subsequent to the mirror and prior to the collimator, orsubsequent to the collimator.

The filter thus creates dark sections in the light field correspondingto leaves (preferably all the leaves) of the collimator. This preventsthe incident light from falling on the leaves and removes the ghostimages at source. By placing the filter in the head, the line can bevery narrow and will be blurred into penumbra at the isocentre. This istherefore a very inexpensive yet effective method of reducing ghosting.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 shows a vertical section through a radiotherapeutic apparatusaccording to the present invention;

FIG. 2 shows a plan view of the filter fitted to the apparatus of FIG.1;

FIG. 3 shows a vertical section through the filter and collimator leavesin more detail;

FIG. 4 shows a section through the filter of FIG. 2; and

FIG. 5 shows alternative placements for the filter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 1, a radiotherapeutic apparatus 10 consists of asource of radiation 12 which is then shaped to a cone beam by anaperture collimator 14. The cone-shaped beam of radiation 16 passesthrough a first filter 18, a second filter 20, both filters beingdesigned to correct the intensity distribution and/or the energy profileof the beam, and then an ion chamber 22 which allows the energy of thebeam to be detected and monitored. The beam 16 then passes through awedge filter 24 which further flattens the intensity distribution acrossthe field of the beam 16, before meeting a multi-leaf collimator 26composed of a plurality of individual leaves 28.

Each leaf of the multi-leaf collimator 26 can be withdrawn or extendedinto and out of the path of the beam 16, in a direction transverse tothe page as shown in FIG. 1. By extending or withdrawing the individualleaves 28 to a different extent, the beam 16 can be given a specificshape as required by the clinical situation.

After the multi-leaf collimator 26, a pair of block collimators 30, 32,can be extended into and out of the beam in the direction transverse tothe direction of the leaf 28 motion, in order to collimate the beam inthat axis. Of course, the block collimator 30, 32 could be replaced witha further multi-leaf collimator arranged transverse to the first blockcollimator 26 in order to provide a more complex shape, or both can bereplaced with other arrangements of multi-collimators as for exampleillustrated in our earlier application No: WO2005/0004987. Further blockcollimators can also be provided, operating in the same direction as themulti-leaf collimators 26 or any further multi-leaf collimators. Thesecover the space between extended leaves; multi-leaf collimators are, toa small extent, slightly more leaky than block collimators and thereforeif all leaves on that particular bank are extended beyond a particularpoint, it is possible to extend a block collimator to that particularpoint so as to provide a reduced background dose.

A light source 34 is provided off to one side of the beam path, and isincident on a mirror 36 that is disposed in the beam path. The mirror 36and the light source 34 are positioned so that, after reflection, thelight field substantially follows the field of the beam of radiation 16.Thus, the light field can replicate the area that will be irradiated bythe beam 16. Prior to operation of the radiation source 12, therefore,the light source therefore can be activated so as to verify thepositioning of the patient within the beam field, for example by way ofa set of cross-hairs, and the operation of the multi-leaf collimator 26can also be checked visually.

A filter 37 is placed in the path of the light beam, between the lightsource 34 and the mirror 36. FIG. 2 shows the filter in the direction ofthe arrow II on FIG. 1. A plurality of parallel dark lines 38 areprovided on the filter 37, and these create a shadow or series ofshadows within the light beam. The filter 37 is adjustable as to itsfine position, and can therefore be adjusted so that the shadows 38 fallon the sides of the MLC leaves 28 as shown in FIG. 3. According to FIG.3, light rays 40, 42 are allowed to pass between the dark lines 38 ofthe filter 37, to fall on the upper edges of the MLC leaves 28. Thus, ifthe leaf 28 is extended, light that would fall on the leaf sides fromwhich reflections are produced is blocked.

FIG. 4 shows a section through the filter 37. In this embodiment, thefilter is formed from a very thin layer of brass, typicallyapproximately 0.5 mm thick. This is etched using a ferric chloride etch,akin to that used for printed circuit boards, in order to form a seriesof slots 46. In those areas where a slot 46 is not formed, this leavesan area of solid material 38 which defines a dark area of the filter.Various alternative methods could be used for defining the filter 37.For example, the slots 46 could be laser-cut, the filter could be asuitably printed or engraved pattern on a transmissive material such asglass, Perspex or other substantially optically transparent polymericmaterial, wires could be held under tension in appropriate locations bya suitable frame, suitable masks could be grown chemically, orotherwise.

FIG. 5 shows an alternative apparatus corresponding to that of FIG. 1.Like reference numerals have been used to denote like parts and will notbe described further. As can be seen, the filter 38 can be positioned inone of a range of alternative positions. In a first alternative, thefilter can be placed very close to the light source, at 48. In thiscase, the filter is likely to be extremely fine and could be embodied asa suitable diffraction grating in order to create a diffraction patternequivalent to the image produced by the filter 38. In a furtheralternative, the filter 38 could be placed between the mirror and themulti-leaf collimator 26, i.e. at 50. Other alternative positions areimmediately after the multi-leaf collimator 26 at 52 or after the blockcollimators 30, 32 at 54.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

The invention claimed is:
 1. A radiotherapeutic apparatus comprising asource of therapeutic radiation, a source of visible light arranged tocast a light field corresponding to a beam of radiation, and a multileafcollimator for shaping the beam of radiation, wherein an optical filtercomprising an alternating plurality of linear dark and light sections isdisposed in the path of a visible light beam, the linear dark sectionscorresponding to leaves of the collimator and so that the shadows madeby the linear dark sections fall on the sides of the leaves to blocklight from falling on the leaf sides.
 2. The radiotherapeutic apparatusaccording to claim 1 including a mirror to deflect the path of thevisible light beam to correspond to that of the radiation beam.
 3. Theradiotherapeutic apparatus according to claim 2 in which the filter isdisposed in the path of the visible light beam prior to the mirror. 4.The radiotherapeutic apparatus according to claim 2 in which the filteris disposed in the path of the visible light beam subsequent to themirror.
 5. The radiotherapeutic apparatus according to claim 1 in whichthe filter is disposed in the path of the visible light beam prior tothe collimator.
 6. The radiotherapeutic apparatus according to claim 1in which the filter is disposed in the path of the visible light beamsubsequent to the collimator.
 7. The radiotherapeutic apparatusaccording to claim 4 in which the filter is disposed in the path of thevisible light beam prior to the collimator.
 8. The radiotherapeuticapparatus according to claim 4 in which the filter is disposed in thepath of the visible light beam subsequent to the collimator.